The present invention relates to a magnetic disk drive and is especially adapted for a magnetic disk drive fitted with a control portion for providing control to place an object to be controlled including a magnetic head in position.
In a magnetic disk drive, a magnetic head follows a track of interest on a surface of a rotating magnetic disk to record (write) and play back (read out) data. In such a magnetic disk drive, the magnetic disk that is a recording medium is fixed to the rotating shaft of a spindle motor. The magnetic disk is driven by the spindle motor and rotates at a determined rotational frequency. Furthermore, a pivot bearing is mounted radially outside the magnetic disk held to the rotating shaft of the spindle motor and parallel to the shaft of the spindle motor. The magnetic head is fixed to the front end of a carriage which is swingably mounted to the pivot bearing. The carriage is driven by a voice coil motor (VCM) and swings about the pivot bearing. That is, the magnetic head is moved radially over the surface of the magnetic disk by swinging movement of the carriage, arrives at a target track, and follows the track to record and play back data.
The magnetic head is moved by the VCM. The magnetic head detects a position signal recorded on a servo sector on the magnetic disk and knows the present position. The position signal detected by the magnetic head is amplified by a head signal amplifier and demodulated into a servo signal by a servo signal demodulator. The servo signal becomes a digital position signal via an A/D converter and is accepted via a bus into an MPU that is a control portion.
The A/D converter, a D/A converter, the MPU, a ROM, a RAM, and an interface controller are connected with this bus. Various kinds of control programs executed by the MPU are stored in the ROM. Furthermore, parameters necessary for various kinds of control are stored.
The position signal accepted into the MPU is processed by the MPU. A VCM control signal is created by the following method. First, the interface controller receives a command of a host-side controller and issues an access request for recording and playback to the MPU. When the command making a request of recording and playback of data is issued, the MPU implements a positioning method recorded in the ROM and creates an optimum VCM control signal according to the distance from the present head position indicated by the accepted position signal to the target position. The created VCM control signal is output to the bus and becomes a power amplifier control signal through the D/A converter. The signal is input to a power amplifier. The input power amplifier control signal is converted into an electrical current by the power amplifier and applied to the VCM. According to the applied electrical current, the VCM produces a driving force for the actuator to place the magnetic head into a target position.
In magnetic disk drives that are external storages for computers, the track width is reduced year by year in order to increase the storage capacity. In order to place the magnetic head in position accurately relative to the track width, it is necessary to improve the positioning accuracy in pursuit to the track width that is reduced year by year. It is necessary that this be achieved in a high-speed operation. Vibrations synchronized to rotation of the disk that are factors deteriorating the positioning accuracy and vibrations by the effects of wind produced inside the apparatus, and so on deteriorate the positioning accuracy at a certain frequency. For example, if the center of rotation of the magnetic disk and the center of the track deviate, the magnetic head must vary its position radially to follow the track even when the magnetic head follows the same track. The situation is the same when the track is not a genuine circle but is distorted. In this case, motion of the magnetic head becomes vibrations of a frequency synchronized to rotation of the magnetic disk. The accuracy of pursuit at this frequency deteriorates. Also, as the magnetic head rotates, wind is produced near the surface of the magnetic disk. This wind produces vibrations of the magnetic head, deteriorating the positioning accuracy. The vibrations occur at a certain frequency that is affected by the structure of the magnetic head or carriage and by the rotational frequency of the magnetic disk.
As mentioned previously, vibrations of the position signal synchronized to rotation of the disk and vibrations of the mechanical system are factors deteriorating the positioning accuracy. These appear as vibrations of the position signal. Control that sufficiently suppresses such vibrations is necessary. A prior art technique for this is available, for example, in JP-A-5-298842 (Patent Reference 1). A technique of placing the magnetic head in position in pursuit of the eccentric component contained in the positional information is disclosed. In the magnetic disk drive of this Patent Reference 1, a method is shown which consists of creating a control signal for suppressing the vibrations using a mathematical model and a low-pass filter limiting the band and applying the control signal as a servo control signal to place the magnetic head in position. The mathematical model has resonant points at vibrational frequencies due to disk rotation, external disturbances, and so on.
Other prior art techniques are described in JP-A-2002-352534 (Patent Reference 2) and JP-A-2003-331543 (Patent Reference 3). Methods of designing a resonant filter are shown, the filter achieving a mechanical resonant mode having a resonant point close to the frequency at which the positioning accuracy is improved. In the magnetic disk drive of Patent Reference 2, a method of coupling a resonant filter having stable phase conditions in series to the object to be controlled is shown. In the magnetic disk drive of Patent Reference 3, a method of coupling a resonant filter having stable phase conditions in parallel to an object to be controlled is shown.